Fix CI errors (#532)

We hastly merged in the changes in the previous pull requests. This
patch fixes the CI errors.

aas_core_codegen/protobuf/common.py

@@ -1,12 +1,12 @@
 """Provide common functions shared among different ProtoBuf code generation modules."""
 
 import re
-from typing import List, cast, Optional
+from typing import List, cast
 
 from icontract import ensure, require
 
 from aas_core_codegen import intermediate
-from aas_core_codegen.common import Stripped, assert_never
+from aas_core_codegen.common import Stripped, assert_never, Identifier
 from aas_core_codegen.protobuf import naming as proto_naming
 
 
@@ -147,7 +147,7 @@ def generate_type(type_annotation: intermediate.TypeAnnotationUnion) -> Stripped
                 # We have to add the suffix ``_choice`` since this field points
                 # to one of the concrete descendants of the class as well as
                 # the concrete class itself.
-                message_name += "_choice"
+                message_name = Identifier(message_name + "_choice")
 
             return Stripped(message_name)
 

aas_core_codegen/protobuf/structure/_generate.py

@@ -9,13 +9,11 @@
     Tuple,
     cast,
     Union,
-    Set,
 )
 
 from icontract import ensure, require
 
 from aas_core_codegen import intermediate
-from aas_core_codegen import specific_implementations
 from aas_core_codegen.common import (
     Error,
     Identifier,
@@ -403,7 +401,7 @@ def _generate_choice_class(cls: intermediate.ClassUnion) -> Stripped:
         # Protocol Buffers do not support inheritance, so we have to work around that
         # circumstance.
 
-        message_name = proto_naming.class_name(cls.name) + "_choice"
+        message_name = Identifier(proto_naming.class_name(cls.name) + "_choice")
 
     else:
         assert_never(cls)
@@ -440,8 +438,6 @@ def generate(
 
     errors = []  # type: List[Error]
 
-    required_choice_objects = set([])  # type: Set[intermediate.AbstractClass]
-
     for our_type in symbol_table.our_types:
         if not isinstance(
             our_type,